1. Field of the Invention
The invention relates to a furnace head for a heavy-current resistance furnace of refractory masonry and at least one cooled electrode inserted into the masonry.
2. Description of the Prior Art
Heavy-current resistance furnaces are furnaces, in which the charge stacked between the electrodes is heated to temperatures of 2000.degree. C. and above through the direct passage of current at high current intensities. Typical representatives of this type are furnaces for graphitizing carbon products which furnaces generally consist of a rectangular furnace bed equipped with granular refractory materials, furnace heads at the end faces and movable side walls. In the Acheson graphitizing furnace, the material to be graphitized is stacked alternatingly with layers of a granular resistor compound between the furnace heads. Also the stack is surrounded by granular insulating material. The heating to the graphitizing temperature of about 3000 K. is accomplished by resistance heating; the electric current is fed to the stack of charge material via the electrodes of graphite inserted into the furnace heads. With increasing temperature, the charge expands uniformly initially, but in steps with the start of the liberation of the sulfur contained in the carbon, and the volume of the charge then decreases with the increasing degree of crystalline order or graphitizing. Volume changes are taken up substantially by the granular resistor material, the packing density of which is changed accordingly so that no major forces released by volume changes of the charge attack at the electrodes. Nevertheless, gaps are formed between the masonary and the electrode, due especially to the different thermal coefficients of expansion and the temperature difference between the masonry and the electrode.
In the graphitizing furnace first proposed by Castner which is often called a longitudinal graphitizing furnace, the material to be graphitized, for instance, in the form of cylindrical carbon bodies is clamped without intermediate layers of granular resistor material between the graphite electrodes of the furnace heads. At least one electrode is movable in the direction of the longitudinal axis of the furnace and is pressed against the strand which rests against the counter-electrode and is formed as a rule of several carbon bodies, for setting a low contact resistance. The length change of the strand in the graphitizing process which in the heating-up phase amounts to about +0.5 to 1 percent and in the cooling-down phase of the furnace about -1 to 1.5 percent is intercepted by the shift of the electrode in the direction of the longitudinal axis of the furnace and opposite to the movement of the strand. A prerequisite for the mobility of the electrode relative to the masonry of the furnace head is sufficient clearance, between the electrode and the masonry.
The parts of the electrodes protruding from the masonry on the side of the furnace head facing away from the furnace are as a rule cooled. In small furnaces, the coolant is sprayed directly on the electrode surface, but in general, cooling plates or cooling staves are used, through which coolants flow. This procedure enables recovery of part of the stored energy as heat in the cooling-down phase of the furnaces. However, the temperature of the electrode is below the critical reaction temperature in only a small area, such that the larger part of the electrode which is not cooled to below the reaction temperature reacts with the air oxygen entering into the furnace through gaps between the masonry and the electrode. As a result of the reaction of the electrode with the air oxygen, the width of the gaps and the rate of burnoff increase and finally, the electrode must be replaced. The detrimental formation of gaps is further promoted by the temperature differences between the masonry and the electrode, due to the differences in thermal conductivity and the generation of Joule heat.